Crystallizing method, thin-film transistor manufacturing method, thin-film transistor, and display device

ABSTRACT

A crystallizing method of causing a phase shifter to phase-modulate a laser beam whose wavelength is 248 nm or 300 nm or more from an excimer laser unit into a laser beam with a light intensity profile having a plurality of inverted triangular peak patterns in cross section and of irradiating the pulse laser beam onto a substrate to be crystallized for crystallization. The substrate to be crystallized is such that one or more silicon oxide films which present absorption properties to the laser beam and differ in the relative proportions of Si and O are provided on a laser beam incident face.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Applications No. 2004-196111, filed Jun. 4, 2004;No. 2004-254283, filed Sep. 1, 2004; No. 2004-286646, filed Sep. 30,2004; and No. 2005-076190, filed Mar. 17, 2005, the entire contents ofall of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a crystallizing method suitable for use in adisplay device, such as a liquid crystal or organic electroluminescence(EL), a thin-film transistor manufacturing method, a thin-filmtransistor, and a display device.

2. Description of the Related Art

The driving circuit for a display device, such as a liquid-crystaldisplay device, has been fabricated by using an amorphous semiconductorfilm formed on a glass substrate. With the expansion of the IT servicesmarket, information handled in this field has been digitized and speededup and therefore display devices have been required to provide higherpicture quality. One of the means for meeting the requirement is suchthat, for example, switching transistors for switching the correspondingpixels are fabricated by using a crystallized semiconductor, therebymaking the switching speed faster, which enables higher picture quality.

One known means for crystallizing an amorphous silicon layer formed on aglass substrate is the excimer laser annealing method (ELA method). Thegrain size of a polycrystalline silicon obtained by the ELA method isabout 0.1 μm. When a thin-film transistor (TFT) is formed in thecrystallized region, a large number of grain boundaries are included inthe channel region of a single thin-film transistor, which gives anelectron field-effect mobility of 200 cm²/Vs. This mobility is muchinferior to that of a MOS transistor fabricated by using a siliconsingle crystal.

The inventors of this invention have developed the industrial techniquefor forming as large a crystal grain as enables a channel region of atleast one thin-film transistor to be formed by irradiating an amorphoussilicon layer with a laser beam. Forming a TFT in a single crystal grainhas no adverse effect from the grain boundaries on transistorcharacteristics which differ from the conventional transistorcharacteristics where the grain boundaries are formed in the channelregion. In addition, the TFT characteristic is improved remarkably and afunctional element, such as a processor, a memory, or a sensor, can beformed. As such a crystallizing method, the inventors have proposed thecrystallizing method described in, for example, W. Yeh and M. Matsumura,Jpn. Appl. Phys. Vol. 41(2002)1909 and M. Hiramatsu, et al., ExtendedAbstracts (The 63^(st) Autumn Meeting, 2002); The Japan Society ofApplied Physics No. 2, P779, 26a-G-2.

The document by W. Yeh and M. Matsumura has described a method ofirradiating a laser beam phase-modulated at a fluence of 0.8 J/cm² to anamorphous silicon film via an SiON/SiO₂ cap layer or an SiO₂ cap layer,causing a Si grain to grow laterally in parallel with the film, whichcrystallizes the amorphous silicon film.

The document by M. Hiramatsu, et al., has described a method ofirradiating a laser beam homogenized and intensity-modulated to anamorphous silicon film via an SiO₂ cap layer, while the substrate isbeing heated, which causes the amorphous film to grow in crystallaterally.

In the method of the document by W. Yeh and M. Matsumura, a Si grainwhose size is equal to or larger than 10 μm can be obtained. Since verysmall grains with small sizes appear near the large grains whose sizeshave become larger, it is expected that the grains with large grain sizeare formed in the same size so as to be relatively uniform (or close) asan overall film structure. The SiON cap film is capable of changing theabsorption spectrum by changing the ratio of oxygen atoms to nitrogenatoms in the film. However, even a film whose optical bandgap is thesmallest (SiNx without oxygen) has about 5 eV (This bandgap energycorresponds to nearly 240 nm in wavelength). Thus, it can be used forKrF laser whose wavelength is 248 nm among excimer lasers, but isdifficult to use for XeCl laser now often used in mass production whosewavelength is 308 nm, because it becomes transparent. This is a problem.

Furthermore, in the methods in the document by W. Yeh and M. Matsumuraand the document by M. Hiramatsu, et al., the substrate has to be heatedto a high-temperature region to make crystal grains larger in its size,preventing the requirement for low-temperature treatment from being met,which is a problem. For example, a conventional crystallizing apparatus100 shown in FIG. 11 is a unit which irradiates a pulse laser beam 105emitted from a KrF excimer laser unit 104 to a substrate 103 to becrystallized which is heated to a high-temperature region by a heater102 built in a table 101, thereby crystallizing the substrate. The pulselaser beam 105 is a laser beam passed through an optical system composedof a concave lens 106, a convex lens 107, and a phase shifter 108. Apower supply 110 controlled by a controller 109 supplies electricity tothe heater 102, which has the capability of heating the substrate 103 toa temperature range from 300 to 750 degrees.

Since the substrate heating temperature may exceed, for example, 500degrees, general-purpose glass (e.g., soda glass) or plastic is liableto deteriorate or deform due to heating. Therefore, to use thesematerials as substrates for a liquid-crystal display (LCD),low-temperature treatment is an indispensable condition. Large-screenLCDs have been strongly required to be lighter. Therefore, theirsubstrates tend to be made thinner and therefore are liable to deformdue to heating, which makes low-temperature treatment an indispensablecondition to secure the flatness of the thin substrates.

Furthermore, since heating the substrate 103 increases the powerconsumption, electric power saving particularly required inindustrialization is not satisfied. Moreover, when a laser light sourcewith a wavelength of, for example, 248 nm shorter than 300 nm is used(e.g., refer to Y. Taniguchi, et al., Extended Abstracts (The 51^(st)Spring Meeting, 2004); The Japan Society of Applied Physics and RelatedSocieties No. 2, P929, 28a-ZG-3), light absorption takes place in anoptical system composed of a concave lens 106, a convex lens 107, aphase shifter 108, and a mirror. In the optical system, heatcorresponding to the amount of light absorbed is generated. In the heatgeneration, the temperature rises with time, which results in theblurring of the focal point of the lens and the displacement of thecrystallization position. The displacement leads to a shift in thecrystallization region. When a transistor circuit is formed, the channelregion is formed off the crystallization region in the exposure process,wit the result that the yield becomes worse in the mass productionprocess. Furthermore, in a display device, it is understood that displayirregularity and color nonuniformity occur, which causes poor display.

BRIEF SUMMARY OF THE INVENTION

It is, accordingly, an object of the present invention to provide acrystallizing method capable of realizing crystallization with a largegrain size even at room temperature, achieving electric power saving,and reducing the occurrence of the displacement of crystallization, athin-film transistor manufacturing method, a thin-film transistor, and adisplay device.

According to an aspect of the present invention, use of a laser beamwith wavelength of the lowest light absorption possible in acrystallizing optical system prevents the optical system from generatingheat and reduces the occurrence of the displacement of crystallization.The present invention provides a crystallizing method of achievingcrystallization with a large grain size at relatively low electric powerby providing at least one SiOx film (x is less than 2) on the laser beamincident face of a non-single-crystal semiconductor film and causing apart of the crystallizing laser beam to be absorbed. The presentinvention further provides a thin-film transistor manufacturing method,a thin-film transistor, and a display device.

According to an aspect of the present invention, there is provided acrystallizing method of irradiating a laser beam onto the incident faceof a non-single-crystal semiconductor film to crystallize thesemiconductor film, the laser beam having a light intensity profileincluding a plurality of inverted triangular peak patterns in crosssection, the crystallizing method comprising providing a cap film whichpresents absorption properties to the laser beam, on the laser beamincident face of the non-single-crystal semiconductor film.

According to another aspect of the present invention, there is provideda crystallizing method of irradiating a laser beam onto the incidentface of a non-single-crystal semiconductor film to crystallize thesemiconductor film, the laser beam having a wavelength of 300 nm or moreand a light intensity profile including plural types of invertedtriangular peak patterns in cross section, the crystallizing methodcomprising providing a cap film which presents absorption properties tothe laser beam, on the laser beam incident face of thenon-single-crystal semiconductor film.

According to still another aspect of the present invention, there isprovided a crystallizing method of irradiating a laser beam onto theincident face of a non-single-crystal semiconductor film to crystallizethe semiconductor film, the laser beam having a wavelength of 300 nm ormore and a light intensity profile including plural types of invertedtriangular peak patterns in cross section, the crystallizing methodcomprising providing at least one layer of SiOx film on the laser beamincident face of the non-single-crystal semiconductor film, x being lessthan 2. Preferably the x is 1.4 to 1.9, and more preferably it is 1.4 to1.8.

According to still another aspect of the present invention, there isprovided a crystallizing method of irradiating a laser beam onto theincident face of a non-single-crystal semiconductor film to crystallizethe semiconductor film, the laser beam having a wavelength of 300 nm ormore and a light intensity profile including a plurality of invertedtriangular peak patterns in cross section, the crystallizing methodcomprising providing a silicon dioxide film and at least one layer ofSiOx film on the laser beam incident face of the non-single-crystalsemiconductor film, x being less than 2. Preferably the x is 1.4 to 1.9,and more preferably it is 1.4 to 1.8.

According to still another aspect of the present invention, there isprovided a crystallizing method of irradiating a laser beam onto theincident face of a non-single-crystal semiconductor film to crystallizethe semiconductor film, the laser beam having a wavelength of 300 nm ormore and a light intensity profile including plural types of invertedtriangular peak patterns in cross section, the crystallizing methodcomprising providing at least one layer of silicon oxide films differingin the relative proportions of Si and O from silicon dioxide film on thelaser beam incident face of the non-single-crystal semiconductor film.

According to still another aspect of the present invention, there isprovided a crystallizing method of irradiating a laser beam onto theincident face of a non-single-crystal semiconductor film to crystallizethe semiconductor film, the laser beam having a wavelength of 300 nm ormore and a light intensity profile including plural types of invertedtriangular peak patterns in cross section, the crystallizing methodcomprising providing at least one layer of silicon dioxide film and atleast one layer of silicon oxide film differing in the relativeproportions of Si and O from silicon dioxide film on the laser beamincident face of the non-single-crystal semiconductor film.

According to still another aspect of the present invention, there isprovided a thin-film transistor manufacturing method comprising a stepof forming at least one layer of SiOx film (x is less than 2) on thelaser beam incident face of a non-single-crystal semiconductor filmprovided on a substrate; a step of irradiating a homogenized pulse laserbeam whose wavelength is 248 nm or 300 nm or more onto thenon-single-crystal semiconductor film via the SiOx film, allowing theSiOx film not only to absorb a part of the pulse laser beam and generateheat but also to melt the irradiated part of the non-single-crystalsemiconductor film, and forming a crystallization region in thenon-single-crystal semiconductor film after the pulse laser beam is cutoff; and a step of forming a thin-film transistor aligned with thecrystallization region.

According to still another aspect of the present invention, there isprovided a substrate to be crystallized, comprising a non-single-crystalsemiconductor film provided on a substrate; and at least one layer ofSiOx film provided on the laser beam incident face of thenon-single-crystal semiconductor film, x being less than 2. Preferablythe x is 1.4 to 1.9, and more preferably it is 1.4 to 1.8.

According to still another aspect of the present invention, there isprovided a substrate to be crystallized, comprising a non-single-crystalsemiconductor film provided on a substrate; and one or more siliconoxide films differing in the relative proportions of Si and O providedon the laser beam incident face of the non-single-crystal semiconductorfilm.

According to still another aspect of the present invention, there isprovided a thin-film transistor comprising a channel region, part of orall of a source region, and part of or all of a drain region which areprovided in a crystallization region fabricated by the crystallizingmethod of claim 1; a gate insulating film formed on the channel region,source region, and drain region; and a gate electrode formed on the gateinsulating film.

According to still another aspect of the present invention, there isprovided a display device comprising a channel region of thin-filmtransistors for switching pixels which are formed in a crystallizationregion fabricated by the crystallizing method of claim 1.

In this specification, “non-single-crystal semiconductor film” is a thinfilm to be crystallized, such as amorphous semiconductor (e.g., anamorphous silicon film), polycrystalline semiconductor (e.g., apolysilicon film), or a composite structure of these.

At least one layer of the silicon oxide films is a film which presentslight absorption properties to the laser beam. The silicon oxide filmshave a film thickness of 100 nm to 1500 nm. The laser beam is a pulselaser beam modulated so as to have a light intensity profile with arepetitive pattern where light intensity alternates between monotonousincrease and monotonous decrease. The laser beam is homogenized in lightintensity before entering the phase shifter.

In the present invention, “crystallization” means a crystal grows fromthe nucleus when the film to be crystallized is melted and solidified.

According to aspect of the present invention, it is possible to providea crystallizing method capable of realizing crystallization with a largegrain diameter even at room temperature, achieving electric powersaving, and reducing the occurrence of the displacement ofcrystallization, a thin-film transistor manufacturing method, asubstrate to be crystallized, a thin-film transistor, and a displaydevice.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the general description given above and the detaileddescription of the embodiments given below, serve to explain theprinciples of the invention.

FIG. 1 is a block diagram showing the configuration of a crystallizingapparatus to help explain a crystallizing method according to thepresent invention;

FIGS. 2A to 2D shows a configuration of the phase shifter of FIG. 1 tohelp explain a light intensity profile modulated by the phase shifter,FIG. 2A is a plane view of the phase shifter, FIG. 2B is across-sectional view of FIG. 2A, FIG. 2C shows a light intensity profileof the laser beam passed through the phase shifter and phase-modulated,and FIG. 2D shows a light intensity profile of the laser beam of FIG. 2Cthree-dimensionally;

FIGS. 3A and 3B are cross-sectional views to help explain the substrateto be crystallized of FIG. 1;

FIG. 4 is a diagram to help explain the optical system of thehomogenizer of FIG. 1;

FIGS. 5A and 5B are diagrams to help explain the optical characteristicof the phase shifter of FIG. 1;

FIG. 6 shows an SEM image to help explain the surface shape of thecrystallization region of the substrate to be crystallized by thecrystallizing apparatus of FIG. 1;

FIG. 7 is a characteristic diagram showing the dependence of SiH₄ gasconcentration during deposition for the second cap insulating film ofFIGS. 3A and 3B to obtain the desired light absorption properties;

FIG. 8 is a characteristic curve diagram to help explain the energysaving effect by the second cap insulating film having the lightabsorption properties of FIGS. 3A and 3B;

FIG. 9 is a cross-sectional view to help explain the process of forminga thin-film transistor in a crystallization region crystallized by thecrystallizing apparatus of FIG. 1;

FIG. 10 is a perspective view of a display device to help explain theprocess of forming a display device using the thin-film transistor ofFIG. 8;

FIG. 11 is a diagram to help explain a conventional crystallizingapparatus;

FIG. 12 shows a transmission spectrum when the concentration of SiH₄ gasused during deposition of the SiOx film of FIGS. 3A and 3B is changed;

FIG. 13 is a characteristic curve diagram showing the energy savingeffect by the second cap insulating film having the light absorptionproperties of FIGS. 3A and 3B in comparison with the characteristic of aconventional cap film;

FIG. 14 shows the relationship between the length of lateral crystalgrowth of the amorphous semiconductor film of the substrate to becrystallized of FIGS. 3A and 3B and the temperature of the amorphoussemiconductor film;

FIGS. 15A to 15D shows the light absorption effect by the second capinsulating film of FIGS. 3A and 3B in comparison with the characteristicof a conventional cap film;

FIG. 16 shows the relationship between the thickness of the cap film ofthe substrate to be crystallized of FIGS. 3A and 3B and the amount oflight contributing to annealing in comparison with the characteristic ofa conventional cap film; and

FIG. 17 is a characteristic curve diagram of the time from immediatelyafter a laser is irradiated until the amorphous semiconductor filmreaches 1000 degrees with respect to the film thickness of the first andsecond films of FIG. 3A.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, referring to the accompanying drawings, a preferredembodiment of the present invention will be explained.

First, the technical terms used in this specification are defined asfollows. “Lateral growth” means that, when a film to be crystallized ismelted and solidified, the growth of a crystal grain progresseslaterally along the film surface. “Input fluence” is a measurerepresenting the energy density of a laser beam for crystallization. Itis obtained by integrating the amount of one shot pulse laser beamenergy per unit area. Specifically, it means the average light intensityof a laser beam measured at a light source or in the irradiation region(irradiation field).

“Phase shifter” is an example of a phase modulation optical system. Itis a spatial intensity modulation optical element for modulating thephase of the laser beam and is distinguished from a phase shift maskused in the exposure step in the photolithographic process. For example,a phase shifter is such that a step is formed in a quartz substrateacting as a transparent medium. The desired step on desired position ofthe phase sifter is formed into such a size as phase of incident lightshifts at a specific phase angle, for example, at 180 degrees by such aprocess as etching.

“Projection lens” is an optical system for projecting an image formed byphase shifter onto the surface of the substrate. When the irradiationsize is small, a telecentric lens is generally used. Use of atelecentric lens makes it possible to prevent the size of the projectedimage from changing even if the distance between the substrate and thelens changes a little. Therefore, a singe-sided telecentric lens systemwhich is parallel only on the substrate side or a double-sidedtelecentric lens which is parallel on both of the substrate side and thelight source side is used. In a mass production apparatus, a long,narrow beam, or a “longitudinal” beam, is often used as a irradiationbeam. Although not shown in the specification, this can be realized byusing a “hog-backed” projection lens which has a lens structure oftelecentric et al. only on the short sides, preventing the lens effecton the long sides.

In this embodiment, using a laser beam with the lowest light absorptionpossible for a crystallization optical system prevents the opticalsystem from generating heat even with a high-energy laser beam, whichdecreases the occurrence of the displacement of the crystallizationposition. The embodiment is related to a crystallizing method ofcrystallizing a large-size grain with the laser beam energy density forthe crystallization with relatively low electric power by providing atleast a silicon dioxide film and at least one layer of SiOx film (x isless than 2) on the laser beam incident face of a non-single-crystalsemiconductor film and causing a part of the crystallizing laser beam tobe absorbed. First, the configuration of a projection crystallizingapparatus to help explain the crystallizing method will be described,referring to FIG. 1.

The crystallizing apparatus 1 comprises an excimer laser unit 2 whichemits a laser beam whose wavelength is 300 nm or more. The crystallizingapparatus 1 further comprises an concave lens 3, a convex lens 4, ahomogenizer 5, a phase shifter 6, a projection lens 7, a table 9 onwhich a substrate to be crystallized is placed, an XYZθ stage 10, and acontroller 11, which are provided on the optical axis of the laser unit2 in that order. That is, the crystallizing apparatus 1 irradiates apulse laser beam 12 onto the substrate 8 on the table 9 by means of thecrystallizing optical system composed of the concave lens 3, convex lens4, homogenizer 5, phase shifter 6, and projection lens 7.

The excimer laser unit 2 which emits a laser beam whose wavelength is300 nm or more outputs a laser beam with low light absorption in thecrystallizing optical system composed of the concave lens 3, convex lens4, homogenizer 5, phase shifter 6, and projection lens 7. For example,an XeCl excimer laser unit which emits a laser beam whose wavelength is308 nm is best suited for the excimer laser unit 2.

In the outgoing optical path of the excimer laser unit 2, there isprovided an attenuator (not shown) which adjusts the amount of laserlight to a specific light fluence. In the outgoing optical path of theattenuator, the homogenizer 5 is provided via the concave lens 3 andconvex lens 4. The homogenizer 5 has the function of leveling a pulselaser beam 12 in an irradiation region. That is, the homogenizer 5 is anoptical system which homogenizes the incident angle and the lightintensity to the phase shifter 6 of the pulse laser beam 12 passingthrough the homogenizer 5.

The homogenized pulse laser beam 12 is further subjected to, forexample, 180° phase shift at the phase shifter 6 shown in FIG. 2A. Thephase shifter 6 is made of a transparent medium. The phase shifter 6with a plurality of linear steps 6 a arranged in parallel as shown inFIGS. 2A and 2B produces phase differences in the pulse laser beam 12 atthe steps 6 a. The pulse laser beam 12 is phase-modulated due to thephase differences, which subjects the pulse laser beam 12 to lightintensity modulation. As a result, a light intensity profile BP (BeamProfile) with a repetitive pattern where monotonic increase andmonotonic decrease are alternated as shown in FIG. 2D is formed in theirradiation section. In the embodiment, the spacing W between phaseshifters 6 is set to 100 μm.

FIG. 2A is a plane view of the phase shifter 6, FIG. 2B is across-sectional view of FIG. 2A, FIG. 2C is a waveform diagram to helpexplain light intensity profile of the pulse laser beam 12 passedthrough the phase shifter 6, and FIG. 2D is a perspective view of FIG.2C. The light intensity profile BP (Beam Profile) of FIG. 2C takes theform of a V-shaped groove light intensity profile of FIG. 2D when beingshown three-dimensionally in a perspective view. Such a phase shifter 6is a area ratio (duty) modulation phase shifter 6. The duty-modulationpattern is formed by changing the area of the surface of the steps 6 aas shown in FIG. 2A to obtain a light intensity profile having aplurality of peak patterns whose cross section is inverted triangular asshown in FIG. 2C. In the light intensity profile of the invertedtriangular peak patterns in cross section, the pitch height (PH) is thesame and the pitch width (PW) is also the same.

The pulse laser beam 12 subjected to light intensity modulation at thephase shifter 6 enters the projection lens 7. The projection lens 7 isprovided so as to form an image of the phase shifter 6 on the topsurface of the substrate to be crystallized 8. The projection lens 7 isan optical system which provides the image at the same magnification orreduces the image at a reduction ratio of, for example, ⅕. For example,a projected image corresponding to an area 100 μm² of the surface of thesteps 6 a of the phase shifter 6 is 4 μm².

FIG. 3A shows the configuration of the substrate to be crystallized 8.The substrate is characterized in that the cap film absorbs a part ofthe crystallizing laser beam. The cap film which presents lightabsorption properties to the crystallizing laser beam makes it possibleto obtain an equivalent lateral growth distance with a low input fluencefor crystallization.

Specifically, the substrate to be crystallized 8 is a structure obtainedby stacking a substrate protecting film 16, a non-single-crystalsemiconductor film, such as an amorphous semiconductor film 17, a firstcap insulating film 18, and a second cap insulating film 19 one on topof another in that order on a substrate 15 made of insulating materialor semiconductor, such as a silicon substrate. A cap film 20 in theembodiment is composed of the first cap insulating film 18 and thesecond cap oxide film 19 with light absorbability. The second cap oxidefilm 19 of light absorbability is, for example, at least one layer ofSiOx film (x is less than 2) or is at least one layer of silicon oxidefilm differing in the relative proportions of Si and O from silicondioxide film.

The substrate protecting film 16 is a material having the effect ofpreventing impurities from infiltrating from the substrate 15 andstoring heat generated in the crystallizing process of the amorphoussemiconductor film 17. The substrate protecting film 16 is, for example,a silicon dioxide film of 1000 nm in thickness. The substrate protectingfilm 16 may be composed of not only one layer but also two layers. Toprevent impurities from infiltrating from the substrate 15, theformation of an SiNx layer on the substrate 15 and further an SiO₂ layeris particularly effective. Moreover, the substrate protecting layer 16may be such that a silicon dioxide film (SiO₂) is formed to a thicknessof, for example, 30 nm on a thermally-oxidized film. The amorphoussemiconductor film 17, which is a film to be crystallized, is made ofamorphous silicon of, for example, 50 nm to 200 nm in film thickness.

The first cap insulating film 18 is composed of a silicon dioxide filmof, for example, 20 to 200 nm or 30 nm in film thickness. FIG. 3B showsa stacked state in a case where the first cap insulating film 18 isabsent or very thin. In FIG. 3A, since the second cap oxide film 19 withlight absorbability is, for example, at least one layer of SiOx film (xis less than 2), the interface can be unstable when the SiOx film isstacked directly on the amorphous semiconductor film 17. To avoid thispossibility, the second cap insulating film 18 made of a chemicallystable silicon dioxide film has been inserted. The film 18 is not alwaysneeded. If needed, it may sometimes be a very thin film. Therefore, forexample, in addition to the structure of FIG. 3A, the structure of FIG.3B may be used.

The second cap insulating film 19, which is a light absorption film thatabsorbs a part of the laser beam 12 and that is increased intemperature, is composed of a silicon oxide film of, for example, 500 nmin film thickness. It is desirable that the film thickness of thesilicon oxide film should be in the range of 100 to 1500 nm, from theviewpoints of light-absorption property and heat storage characteristic.If the film thickness decreases below 100 nm, the amount of heatgenerated by the light absorption of the silicon oxide film runs shortand therefore the amount of the storaged heat becomes insufficient,which makes it impossible to obtain large crystal grains of a desiredsize. If the film thickness exceeds 1500 nm, the amount of lighttransmitted decreases, making it difficult for the pulse laser beam withsufficient fluence to reach the amorphous semiconductor film 17 to becrystallized, which makes it impossible to achieve the object ofcrystallization sufficiently.

Next, the embodiment of the method of manufacturing the substrate to becrystallized 8 having such a double-structure cap film 20 will beexplained more concretely.

On the substrate 15, such as an insulating substrate made of, forexample, a glass substrate, an insulating layer serving as the substrateprotecting film 16 is formed. Not only a glass substrate, a quartzsubstrate, and a plastic substrate but also a metal substrate, a siliconsubstrate, and a ceramic substrate on whose surface an insulating filmis formed may be applied to the substrate 15. It is desirable that a lowalkali glass substrate, such as a Corning #1737 substrate, should beused as the glass substrate. The substrate protecting film 16 is formedby growing a silicon dioxide film to a film thickness in the range of 50to 2000 nm, for example, to a film thickness of 100 nm, by plasmaenhanced chemical vapor deposition method.

On the substrate protecting film 16, an amorphous silicon film is formedas the amorphous semiconductor film 17. In a method of forming anamorphous silicon film, an amorphous Si film is formed to a filmthickness of 200 nm by, for example, plasma enhanced chemical vapordeposition method. On the amorphous Si film, a cap film 20 is formed.

In the cap film 20, a silicon dioxide film is formed as the first capinsulating film 18 on the amorphous Si film. The silicon dioxide film isformed by plasma enhanced chemical vapor deposition method using, forexample, SiH₄ and N₂O as material gases. The silicon dioxide film thatwas close to the stoichiometric composition ratio has a film thicknessof 30 nm. In addition, on the first cap insulating film 18, the secondcap insulating film 19, for example, a silicon oxide film, is formed asa light absorption layer.

The silicon oxide film is formed by plasma-enhanced chemical vapordeposition method using, for example, SiH₄ and N₂O as material gases.The silicon oxide film is formed into a film thickness of 500 nm,differing in flow rate ratio from the silicon dioxide film. Then, thethin films 16 to 19 formed on the substrate 15 are subjected todehydrogenation. The dehydrogenation process is the process of heatingin an atmosphere of nitrogen at 570 degrees for two hours. In this way,the substrate to be crystallized 8 is formed. On the top surface of thesubstrate 8, for example, a silicon oxide film differing in the relativeproportions of Si and O from silicon dioxide film is provided as a lightabsorption layer.

The cap film 20 with light absorption properties needs to have such afilm thickness as assures the function of absorbing a part of theincident pulse laser beam 12 homogenized and storing the generated heat.The time of the heat storage is a time that enables a large graingrowth.

The table 9, which is mounted on the XYZθ stage 10, can be not onlymoved along the X axis and the Y axis on a horizontal plane and furtheralong the Z axis perpendicular to the horizontal plane but also rotatedon the Z axis through an angle of θ. A power supply circuit for the XYZθstage 10 is connected to the output section of the controller 11 in sucha manner that the X-axis driving mechanism, Y-axis driving mechanism,Z-axis driving mechanism, and θ-rotation driving mechanism arecontrolled independently. A power supply circuit for the excimer laserunit 2 is connected to the output section of the controller 11 in such amanner that the oscillation timing, pulse interval, and output magnitudeof the laser beam 12 are controlled.

Next, referring to FIG. 4, the optical system of the homogenizer 5 willbe explained concretely. The same parts as those in FIG. 1 are indicatedby the same reference numerals. To avoid repetitive explanation, adetailed explanation of them will be omitted. The optical systemincludes as the excimer laser light source 2 an excimer laser lightsource which emits an excimer laser pulse laser beam whose wavelength isequal to or longer than 300 nm, for example, 308 nm of XeCl excimerlaser. The laser beam emitted from the excimer laser light source 2 isexpanded via the optical systems 3, 4 composed of beam expanders andthen enters the homogenizer 5. The homogenizer 5 is composed of a firstcylindrical lens 25, a first condenser optical system 26, a secondcylindrical lens 27, and a second condenser optical system 28.

The pulse laser beam which has entered the homogenizer 5 enters thefirst cylindrical lens 25. On the back focal surface of the firstcylindrical lens 25, a plurality of light sources are formed. Theluminous fluxes from the plurality of light sources illuminate theincident face of the second cylindrical lens 27 in an overlapping mannervia the first condenser optical system 26. As a result, more lightsources than those on the back focal surface of the first cylindricallens 25 are formed on the back focal surface of the second cylindricallens 27. The luminous fluxes from the light sources formed on the backfocal surface of the second cylindrical lens 27 illuminate the phasemodulation element 6 (phase shifter) in an overlapping manner via thesecond condenser optical system 28.

The first cylindrical lens 25 and first condenser optical system 26constitute a first homogenizer. The first homogenizer homogenizes theincident angle on the phase shifter 6.

The second cylindrical lens 27 and second condenser optical system 28constitute a second homogenizer. The second homogenizer homogenizes thelight intensity (laser fluence) in each position of the surface of thephase shifter 6. In this way, the illumination system irradiates thephase shifter 6 with a beam which has a substantially homogeneous lightintensity profile.

It is desirable that the pulse laser beam 12 entering the phase shifter6 should be homogenized in incident angle by the first cylindrical lens25 and first condenser optical system 26 serving as a homogenizingoptical system (homogenizer) and further homogenized in light intensityby the second cylindrical lens 27 and second condenser optical system28.

Specifically, the pulse laser beam 12 homogenized in incident angle andlight intensity at the homogenizer passes through the phase shifter 6,which produces an ideal light intensity profile where light intensityalternates between monotonous increase and monotonous decrease as shownin FIG. 2C. The light intensity profile BP of FIG. 2C, which has aninverted triangular cross section, has the maximum peak value andminimum peak value at projecting parts and has no flat part. Inaddition, the light intensity profile BP has an equal pitch height PHand an equal pitch width PW. Specifically, since the phase-modulatedhomogenized laser beam includes no high-order vibrational component,when the substrate to be crystallized 8 is irradiated with the pulselaser beam 12, a large crystal grain of a size corresponding to thepitch width W between steps 6 a-6 a of the phase shifter 6 can be grownlaterally in theory. At this time, since thermal energy transferred tothe crystallized film from the cap films by the heat generation effectand heat storage effect resulting from the light absorption of the capfilms, a series of processes of melting->solidification andcrystallization->lateral crystal growth are promoted, which makes thesize of the crystal grain larger. In the light intensity profile BP ofFIG. 2C, as the angle θ of the peak part grows sharper, the filmablation is liable to take place. Therefore, it is desirable that thelight intensity profile should be set so as to make the angle θ of thepeak part as gentle as possible.

While in the embodiment, the projection method of projecting a modulatedbeam profile from the phase shifter 4 onto the substrate 5 has beenexplained, the present invention is not limited to this. For instance,the invention may be applied to a proximity method in which the phaseshifter 4 is provided above the substrate 5 with a specific distancebetween them.

Next, a crystallizing method in the crystallizing apparatus 1 will beexplained concretely. The same parts as those in FIGS. 1 to 4 areindicated by the same reference numerals. A detailed explanation of themwill be omitted to avoid repetition. The controller 11 is controlledautomatically according to a program previously stored. The controller11 transfers the substrate to be crystallized 8 to a predeterminedposition of the table 9 and controls, for example, an electrostaticchuck or a vacuum chuck to temporarily fix the substrate 8. According toa predetermined procedure, the controller 11 positions the substrate 8temporarily fixed.

The controller 11 performs control to oscillate the excimer laser unit2. As a result, the excimer laser unit 2, such as an XeCl excimer laserunit 2, oscillates and emits pulse laser 12. The pulse laser 12 producesa pulse laser beam 50 with a pulse width of, for example, 30 nsec and anirradiate laser fluence of, for example, 1 J/cm². The pulse laser beam12 is diverged by the concave lens 3 and converged by the convex lens 4and enters the homogenizer 5. The homogenizer 5 homogenizes the incidentangle and light intensity of the incoming pulse laser beam 12.

The homogenizer 5 causes the homogenized pulse laser beam 12 to enterthe phase shifter 6. The phase shifter 6 emits a pulse laser beam 12with a light intensity profile which has a plurality of invertedtriangular peak patterns in cross section. The pulse laser beam 12emitted from the excimer laser unit 2 has its light intensity andincident angle homogenized at the homogenizer 5. The homogenized beam 12is then modulated by the phase shifter 6 into a light intensity profilehaving a plurality of inverted triangular peak patterns in crosssection. An image of the light intensity profile is formed by theprojection lens 7 on the substrate 8 to be crystallized. As a result,the amorphous semiconductor film 17 of the image forming section ismelted and, after the laser beam is cut off, is crystallized.

In the crystallization process, the amorphous semiconductor film 17 iscrystallized in the following process in the substrate 8. When the pulselaser beam 12 with a light intensity profile having a plurality ofinverted triangular peak patterns in cross section enters the substrate8 to be crystallized, the light of the pulse laser is absorbed in partby the second cap insulating film 19, which is a light absorption filmprovided on the incident face of the pulse laser beam of the amorphoussemiconductor film 17 of the substrate 8. Most of the remaining part ofthe pulse laser beam 12 enters the amorphous semiconductor film 17 andmelts only the irradiated part immediately in the direction of depth.

The temperature rise of the amorphous semiconductor film 17 at this timeis transferred to the substrate insulating film 16, the first capinsulating film 18 and the second cap insulating film 19, and heat isstored. This heat storage effect and a heat storage effect caused by thepulse laser beam absorption of the second cap insulating film 19 canprevent that irradiated part of the temperature of the amorphoussemiconductor film 17 drops sharply when the pulse laser beam is cutoff. Therefore, these effects make it possible to form a region withlarge grains.

The non-single-crystal semiconductor film 52 is melted immediately inthe direction of depth. When the pulse laser beam is cut off,solidification (or crystallization) is started from the inverse peakpoint at which the fluence becomes the lowest, with the result that acrystal grain grows in a lateral direction (or in a directionperpendicular to the thickness of the film 52). In the crystal growth,since the lateral growth of the crystal grain is promoted by the heatstorage effect of the substrate insulating film 16, first cap insulatingfilm 18, and second cap insulating film 19, the size of the crystalgrain after final solidification becomes larger, which realizes theformation of single crystal over a wide region of the irradiated part.The heat storage effect of the second cap insulating film 19 makes itpossible to obtain an equivalent lateral growth distance with low inputfluence for crystallization.

Such a crystallization process is carried out continuously orintermittently all over a predetermined region of the non-single-crystalsemiconductor film 52 by relatively moving the excimer laser lightsource 2 and the substrate 8 to be crystallized, for example, as aresult of the controller 11 moving the XYZθ stage 10.

After having directed tremendous efforts toward forming closely as largea crystal grain as enables at least one channel region of thin-filmtransistor to be formed, the inventors have come to the conclusion thatit is impossible to make a crystal grain larger closely by theirradiation of a conventional parallel pulse laser beam. Strictly, thecause of this has not been made clear. However, what will be describedbelow is roughly speculated.

The light intensity profile after a parallel pulse laser beam passesthrough one step 6 a of the phase shifter 6 is shown in FIG. 5B. In thelight intensity profile, the components contributable to lateral crystalgrowth are from the first inverse peak wave 91 to the next peak wave 92.High-order waves 93 (vibrations; interference fringes) outside thesepeak waves can be considered to be components suppressing lateralcrystal growth. Therefore, it was found that crystal grains grow in avery short time from inverse peak waves 93 of high-order vibrationsother than the main inverse peak wave 91 attributing to lateral crystalgrowth, producing fine crystal grains, which makes it impossible to makea large grain size uniformly and closely all over the film. That is, itwas found that, since a beam obtained by phase-modulating a parallelpulse laser beam (or an unhomogenized beam) includes high-ordervibrations 93, a crystal grain with a large grain size cannot be formed.

The crystallization can be achieved by irradiating the homogenized pulselaser beam onto the amorphous semiconductor film via a phase modulationoptical system and an insulating film with light absorption propertiesand arranging crystal grains with a large grain size closely. In otherwords, a pulse laser beam with a light intensity profile not includinghigh-order vibrations 93 of FIG. 5B as shown in FIG. 2C is irradiatedonto an amorphous semiconductor film, which makes it possible to arrangecrystal grains with a large grain size closely (uniformly).

In the crystallizing method of the embodiment, a pulse laser beam withan optimized light intensity profile, that is, a pulse laser beam asshown in FIG. 2C from which the effect of high-order vibrations 93 shownin FIGS. 5A and 5B has been removed, is irradiated onto an amorphoussemiconductor film 17 (an amorphous film or a polycrystalline film) tobe crystallized via a silicon oxide film acting as the second capinsulating film 19 serving as a light absorption film differing in therelative proportions of Si and O from silicon dioxide film. As a result,the silicon oxide film increases in temperature all over the film bylight absorption. The heat is stored in the silicon oxide film for aspecific period. The stored heat energy from the silicon oxide film isapplied to the amorphous semiconductor film 17 for a heat storage time,heating the film 17. The heating from the silicon oxide film during theheat storage time lengthens the lateral growth length of the amorphoussemiconductor film 17, with the result that crystal grains with a largegrain size are formed in such a manner they are arranged closely.

Specifically, in the method of the embodiment, a film with large grainsof Si each of which is a single crystal or close to a single crystal canbe obtained at R.T, and using a low input fluence without externallyheating the substrate 8 differently from a conventional method, becauseof the heat supplying effect directly from the cap insulating film as alight absorption film to the laser beam incident force of the a-Sisemiconductor film 17 of the substrate 8.

FIG. 3A is an embodiment of the substrate 8 to be crystallized. Thesubstrate 15 made of insulating material or semiconductor is, forexample, a silicon substrate 48. The substrate protecting film 16 is asilicon dioxide film of 1000 nm in thickness. The amorphoussemiconductor film 17, which is a semiconductor film to be crystallized,is an amorphous silicon film of 50 nm to 200 nm in film thickness. Thefirst cap insulating film 18 is a silicon dioxide film of 30 nm in filmthickness. The second cap insulating film 19, which is a lightabsorption film that absorbs a part of the laser beam 12 and transformthe absorbed beam into heat, is a silicon oxide film of 500 nm inthickness differing in the relative proportions of Si and O from silicondioxide film.

The method of manufacturing the substrate 8 to be crystallized with thecap film 20 will be explained more concretely. The pulse laser beam 50homogenized by the crystallizing apparatus 1 of FIG. 1 wasphase-modulated. The phase-modulated laser beam 50 was irradiated ontothe substrate 8 having the double-structure cap insulating films 18, 19,thereby crystallizing the amorphous silicon film in a lateral growthmanner.

The table 9 was moved by the XYZθ stage 10 along each of the X axis, Yaxis, Z axis, and θ rotation axis, thereby positioning the substrate 8very accurately with respect to the crystallizing optical system.

The controller 11 causes the excimer laser light source 2 to emit apulse laser beam 12 with an irradiation laser fluence of 620 mJ/cm². Thepulse laser beam 12 is magnified at the concave lens 3 and convex lens 4constituting a beam expander. Then, the magnified beam 12 is homogenizedin incident angle at a first homogenizer section composed of thecylindrical lens 25 and condenser lens 26 and then is furtherhomogenized in light intensity at a second homogenizer section composedof the cylindrical lens 27 and condenser lens 28. Furthermore, thehomogenized pulse laser beam 12 is subjected to 180° phase modulation atthe phase shifter 6 having steps 6 a, passes through the projection lens7 and then enters the silicon oxide film 19 on the substrate to becrystallized 8. As a result, the irradiated part of the amorphoussilicon film was melted and dropped in temperature when the pulse laserbeam was cut off. In the temperature dropping process, crystallizationprogressed laterally. The average crystal grain size of the crystalgrain was as long as about 8 μm.

FIG. 6 shows an SEM image of an Si thin film crystallized by thecrystallizing method. As seen from the SEM image, the formation of an Sicrystal grown laterally into a large crystal grain was verified.Moreover, it was verified that the laterally grown Si crystals extendedlaterally very well from the central crystal nucleus and were arrangedclosely.

After one shot irradiation of the pulse laser beam 1, the controller 11translated the substrate 8 by a specific pitch distance automaticallyaccording to a previously stored program, caused the excimer laser unit2 to irradiate another shot of the pulse laser beam 12 onto thesubstrate 8, thereby growing a crystal laterally in the irradiationregion of the amorphous silicon film, which formed a larger crystalgrain of Si crystallization region in the amorphous silicon film. Asimilar operation was repeated, thereby growing crystals one afteranother in a predetermined element formation region of the amorphoussilicon film.

FIG. 7 is a characteristic diagram with the flow rate ratio of silanegas to nitrous oxide gas (N₂O) on the abscissa axis and the absorptioncoefficient (cm⁻¹) of the resulting silicon oxide film on the ordinateaxis. The absorption coefficient corresponds to a wavelength of 248 nmas the wavelength of KrF excimer laser and a wavelength of 308 nm as thewavelength of XeCl excimer laser. FIG. 7 is a characteristic diagramobtained by replacing the KrF excimer laser unit with the XeCl excimerlaser unit in the crystallizing apparatus 1 of FIG. 1.

As seen from FIG. 7, the absorption coefficient can be changed bychanging the flow ratio at each of the wavelengths. The reason for thismay be that, since changing the flow ratio increases the amount of Si inthe second cap insulating film 19 as compared with the ratio of thenumber of silicon atoms to that of oxygen atoms is about 1:2 in thesilicon dioxide film acting as the first cap insulating film 18, theSiO₂ film with very low absorption is changed toward to an a-Si filmwith very high absorption in terms of absorption coefficient.

Furthermore, since the absorption coefficient can be changed at eachwavelength, even if the wavelength of laser beam is changed, setting theabsorption coefficient of the silicon oxide film to a desired valueenables the same effect to be obtained. Although not shown in theembodiment, the optical bandgap of SiO₂ is about 9 eV (137 nm in termsof light wavelength) and the optical bandgap of a-Si is about 1.7 eV(727 nm in terms of light wavelength). Therefore, controlling the flowrate ratio enables the same effect to be obtained for beams whosewavelengths are between them.

The transmission spectrum of the SiOx film is shown in FIG. 12. FIG. 12is a characteristic curve diagram showing the characteristic oftransmittance for a wavelength when the concentration of SiH₄ gas duringdeposition is changed. The SiOx film was formed under the controlling soas to give about 7000 cm⁻¹ to α at each wavelength. In a sample of theembodiment, the lateral growth distance exceeded 8 μm. In contrast, thelateral growth distance of a comparative sample was a little under 3 μm.

From what has been described above, it has been verified that use of themethod of the invention enables large crystal grains (with an averagecrystal grain diameter of 4 to 8 microns) to be grown laterally at ahigh padding density.

FIG. 8 is a characteristic curve diagram showing the energy savingeffect when the cap film is provided with light absorption properties.In FIG. 8, the abscissa axis shows the input fluence (mJ/cm²) of thelaser beam emitted from the excimer laser unit 2 and the ordinate axisshows the lateral growth distance (μm) when the amorphous semiconductorfilm 17 is crystallized. In FIG. 8, not only is the KrF excimer laserunit replaced with an XeCl excimer laser unit, but also the second capfilm 19 is changed as follows in the crystallizing apparatus 1 ofFIG. 1. To obtain a lateral crystal growth distance of about 5 μm, aninput fluence of 650 mJ/cm² is used on the black-circle curve obtainedwhen the second cap insulating film 19 is composed of a silicon dioxidefilm, whereas an input fluence of 450 mJ/cm² is used on the white-boxcurve obtained when the second cap insulating film 19 is composed of aSiOx film.

The input fluence is reduced by about 30%. Therefore, the substrate tobe crystallized 8 using the SiOx cap film of the present invention hasthe energy saving effect. Lowering the energy of the laser beam forcrystallization makes it possible to lower the temperature of heatgenerated by the laser beam at the concave lens 3, convex lens 4,homogenizer 5, phase shifter 6, and projection lens 7 in thecrystallizing apparatus 1, which produces the effect of alleviating thedisplacement of the optical image and the blurring of the focal pointdue to high temperatures. Even when the KrF excimer laser unit is used,in this embodiment, the second cap insulating film 19 is not composed ofa silicon oxide film containing nitrogen. This produces the effect ofeliminating nitrogen atom contamination in the process.

The amount of light reaching the Si surface can be adjusted according tothe value of the absorption coefficient of the cap film 20. Setting thex value of the SiOx film with a suitable absorption coefficient to 2 orless enables lateral crystal growth at a low fluence, regardless of thewavelength of the laser beam from the excimer laser unit 2. Anevaluation of crystallization has shown that both of the crystal grainsize and the crystal orientation had sufficiently acceptable values whenx was in the range of 1.4 to 1.9.

In FIG. 8, the black-circle curve is a crystallization characteristiccurve of the substrate 8 to be crystallized where a silicon dioxide filmwas formed to a film thickness of 200 nm as the second cap insulatingfilm 19. The white-box curve is a crystallization characteristic curveof the substrate 8 to be crystallized where an SiOx film (x is less than2) was formed to a film thickness of 250 nm as the second cap insulatingfilm 19.

In the embodiment, the first cap insulating film 18 has been formed thinand the cap 20 has been composed of the first and second cap insulatingfilms 18, 19, or two layers. If the second cap insulating film 19presents absorption properties to a crystallizing laser beam whosewavelength is 300 nm or more, it is possible to achieve crystallizationwith a large grain size even at room temperature as in the embodiment,which enables a crystallizing method of achieving power saving anddecreasing the occurrence of the displacement of crystal grains.

FIG. 13 is a characteristic curve diagram of the time from immediatelyafter laser irradiation until the amorphous semiconductor film 17reaches 1000 degrees with respect to the input fluence of thecrystallizing pulse laser beam irradiated onto the substrate 8 to becrystallized. The circle-symbol characteristic is a characteristic ofthe substrate 8 to be crystallized where the cap film 20 is composedonly of the first cap film 18, the SiO₂ film is formed to a filmthickness of 200 nm, the amorphous silicon film is formed to a filmthickness of 200 nm, an SiO₂ film is formed to a film thickness of 30 nmas the substrate insulating film 16. The triangle-symbol characteristicis a characteristic of the substrate to be crystallized 8 where an SiO₂film is formed to a film thickness of 30 nm as the first cap film 18, anSiOx film with extinction coefficient k=0.02 to a film thickness of 250nm as the second cap film 18, an amorphous silicon film is formed to afilm thickness of 200 nm, and an SiO₂ film is formed to a film thicknessof 30 nm as the substrate insulating film 16.

The light absorption effect of the cap film 20 composed of the first andsecond cap films 18, 19 makes the solidification time longer. Thesolidification time correspond to the time from when the crystallizingpulse laser beam phase-modulated at the phase shifter 6 enters theamorphous semiconductor film 17, melting the irradiated region of theamorphous semiconductor film 17, which completes the irradiation periodof the pulse laser beam, until the temperature of the melted regiondrops gradually and the position at which the solid-liquid separationtemperature is reached moves in the melted region and the whole regionis solidified. Lengthening the solidification time makes the lateralcrystal growth length longer.

This is shown in FIG. 14. FIG. 14 shows that, if the lateral growth hasprogressed to position X1 at t=t1 after time t=t0, the position at whichthe solidification temperature Ts is crossed moves toward X1, showingthat a continuous crystal grain is obtained at this point in time. Ifthis phenomenon continues until t=t₂, long lateral growth with a singlegrain (in the crystallization region) is achieved. That is, to achievelong lateral growth, the temperature dropping rate should be smallerthan the fastest lateral growth rate. What realizes the smalltemperature dropping rate is the effect of the cap layer 20 having thesecond cap insulating film 19.

It is seen from FIG. 14 that to achieve a long lateral growth requiresthe solidification time of the melted region to be extended. Providingthe cap layer 20 with light absorption properties causes thesolidification time to be extended. This effect will be explained,referring to FIGS. 15A to 15D. FIG. 15A shows the configuration of thesubstrate to be crystallized 8 of FIG. 3A where the cap layer 20 iscomposed only of an SiO₂ layer 18 formed to a film thickness of 130 nm.Specifically, the configuration of the substrate 8 of FIG. 15A is suchthat the film thickness of the SiO₂ film as the first cap layer 18 is130 nm, the film thickness of the amorphous silicon film is 50 nm, andthe film thickness of the SiO₂ film as the substrate insulating film 16is 1000 nm. FIG. 15B shows the temperatures in the direction of depth ofthe substrate 8 to be crystallized 20 nsec (X1) after and 200 nsec (Y1)after the crystallizing pulse laser beam has entered the amorphoussemiconductor film 17 and melted the irradiated region of the amorphoussemiconductor film 17 and the irradiation period of the pulse laser beamhas expired.

FIG. 15C shows the configuration of the substrate to be crystallized 8provided with the first and second cap films 18, 19 shown in FIG. 3A.Specifically, the configuration of the substrate 8 of FIG. 15C is suchthat the film thickness of the SiO₂ film as the first cap layer 18 is 30nm, the film thickness of the SiOx film as the second cap film 19 is 320nm, the film thickness of the amorphous silicon film is 50 nm, and thefilm thickness of the SiO₂ film as the substrate insulating film 16 is1000 nm. FIG. 15D shows the temperatures in the direction of depth ofthe substrate 8 to be crystallized 20 nsec (X2) after and 560 nsec (Y2)after the crystallizing pulse laser beam has been irradiated onto thesubstrate 8 and melted the irradiated region of the amorphoussemiconductor film 17 and the irradiation period of the pulse laser beamhas expired.

FIGS. 15A to 15D shows that the solidification time of the substrate 8provided with only the SiO₂ layer 18 of 130 nm in thickness as the capfilm 20 of FIG. 15A is 200 nsec, whereas the solidification time of thesubstrate 8 provided with the first and second cap films 18, 19 of FIG.15C (FIG. 3A) is extended to 560 nsec. The substrate to be crystallized8 of FIG. 15C (FIG. 3A) is the cap film 20 composed of the first andsecond cap films 18, 19, which makes the amount of heat stored largerand enables a longer solidification time to be secured.

The cap layer 20 having the second cap insulating film 19 has thefunction of storing as much heat as possible and a slow temperaturedropping rate function. The second cap insulating function 19 of the caplayer 20 is composed of a film having light absorption properties. Anexample of the second cap insulating film 19 is SiOx (x<2.0). Ideally,the optical gap can be changed from Si (about 1.1 eV: x=0) to SiO₂(about 9 eV: x=2). The absorption coefficient can be controlled to about0 to 10⁵ cm⁻¹ with the wavelength of the laser used for crystallization,such as excimer laser. Particularly when x is in the range of 1.4≦x≦1.9,a good crystal growth is verified.

FIG. 16 shows characteristic curves of the amount of light contributingto crystallization annealing with respect to the thickness of the caplayer 20 when, for example, an XeCl excimer laser beam whose wavelengthis 308 nm as a crystallizing laser beam. In the characteristic curves,the lower one is a characteristic curve of a conventional cap filmwithout light absorption. The upper curve is a characteristic curve ofthe light absorption effect produced by the SiOx film with k=0.02. Ahigh usability of incident light is shown.

Here, k is an extinction coefficient (the imaginary part of the complexrefractive index). The characteristic curve of FIG. 16 shows the effectof the SiOx film acting as a light absorption layer. As seen from FIG.16, the cap layer 20 increases the usability of the incoming laser beamby the difference between the upper curve and the lower curve.

FIG. 17 is a characteristic curve diagram of the time from immediatelyafter the laser is irradiated until the temperature of the amorphoussemiconductor film 17 reaches 1000 degrees with respect to the filmthickness of the first and second cap films 18, 19 constituting the capfilm 20. In FIG. 17, the white boxes show a characteristic during thetime from immediately after the laser beam is irradiated onto the SiO₂layer serving as the first cap layer 18 until the temperature of theamorphous semiconductor film 17 reaches 1000 degrees. The black circlesshow a characteristic during the time from immediately after the laserbeam is irradiated onto the SiOx layer serving as the second cap layer19 until the temperature of the amorphous semiconductor film 17 reaches1000 degrees.

Next, the configuration of a thin-film transistor (TFT) formed in acrystallized region and a method of manufacturing the thin-filmtransistor will be explained, referring to FIG. 9. A thin-filmtransistor was produced using a substrate 8 to be crystallized which hada semiconductor film formed by the aforementioned crystallizing methodso as to have large crystal grains. The same parts as those in FIGS. 1to 8 and FIGS. 12 to 16 are indicated by the same reference numerals. Adetailed explanation of them will be omitted.

A substrate protecting film 16 is formed on a substrate 15 made ofinsulating material or semiconductor, such as a low alkali glasssubstrate. The substrate protecting film 16 is an insulating filmincluding silicon dioxide (SiO₂) or silicon nitride as a main component,such as a silicon dioxide film of 300 nm in film thickness. It isdesirable that the substrate protecting film 16 should be formed inclose contact with the glass substrate. The substrate protecting film 16is a film which prevents impurities from diffusing from the substrate15, such as a glass substrate to the amorphous semiconductor film.

On the substrate protecting film 16, an amorphous semiconductor film, orthe amorphous semiconductor film 17 made of non-single-crystalsemiconductor, such as an amorphous silicon film, is formed. Theamorphous silicon film is an amorphous Si film of 200 nm in filmthickness formed by, for example, plasma-enhanced chemical vapordeposition method.

On the amorphous silicon film, a cap film 20 with light absorptionproperties is formed, thereby forming a substrate 8 to be crystallized.The pulse laser beam homogenized by the homogenizer 5 of FIG. 4 iscaused to enter the phase shifter 4 and is phase-modulated, therebyproducing a pulse laser beam 12 with a light intensity profile which hasa plurality of inverted triangular peak patterns in cross section ofFIG. 2C. The laser beam 12 is irradiated onto the substrate to becrystallized 15, which completes the crystallization process.

Next, the first and second cap films 18, 19 on the crystallizedamorphous semiconductor film 17 are removed by etching. Then, beingaligned with the crystallized region of the exposed amorphoussemiconductor film 17, a semiconductor circuit, such as a thin-filmtransistor 35 shown in FIG. 9, is fabricated as follows. First, todefine the shape of an active region, patterning is done byphotolithography, thereby forming a Si island with a predeterminedspecific pattern almost corresponding to a channel region 36, a sourceregion 37, and a drain region 38 in the plane visual field. At thistime, the channel region 36 is formed in the crystallized region.

Next, on the channel region 36, source region 37, and drain region 38, agate insulating film 40 is formed. The gate insulating film 40, which ismade mainly of silicon dioxide or silicon oxynitride (SiON), is asilicon dioxide film of 10 to 200 nm in thickness or a siliconoxynitride film of 30 to 500 nm in thickness. In the embodiment, a3-nm-thick silicon dioxide film is provided on the side contactingsilicon. On the silicon dioxide film 41, a silicon oxynitride film 42 isformed to a thickness of 50 nm using silane gas, ammonia gas, andnitrous oxide gas by plasma CVD techniques. On the silicon interfaceside, the silicon dioxide film 41 whose interface state density is lowis used. On the film 41, the SiON film 42 with high permittivity is usedto make leakage current smaller. The present invention is not limited tothis. Use of only one layer does not depart from the spirit of thepresent invention. Although not shown in the embodiment, a film formedby oxidizing the surface of the Si island 39 by oxygen plasma may beused as the silicon dioxide film in the lower part of the gateinsulating film 40.

Next, on the gate insulating layer 40, a conductive layer was formed toform a gate electrode 43. The conductive layer, which was mad mainly ofsuch element as Ta, Ti, W, Mo, or Al, was formed by a known film formingmethod, such as sputtering or vacuum deposition techniques. For exampleMo—W alloy was used. The gate electrode metal layer was patterned byphotolithography, thereby forming a gate electrode 43 with a specificpattern.

Next, with the gate electrode 43 as a mask, impurities were implanted,thereby forming a source region 37 and a drain region 38. For example,to form a p-channel TFT, p-type impurities, such as boron ions, areimplanted by ion implantation techniques. The boron concentration inthis area was set to, for example, 1.5×10²⁰ to 3×10²¹ cm⁻³. In this way,a high-concentration p-type impurity region constituting the sourceregion 37 and drain region 38 of the p-channel TFT is formed. It goeswithout saying that implanting n-type impurities at this time causes ann-channel TFT to be formed.

Next, a heat treatment process is carried out to activate the impurityelements implanted by the ion implantation. The process is carried outby furnace annealing, laser annealing, or rapid thermal annealing. Inthe embodiment, the activating process was performed by furnaceannealing techniques. It is desirable that the heating process should becarried out in the temperature range of 300 to 650 degrees in anatmosphere of nitrogen. In the embodiment, the heat treatment wascarried out at 500 decrees for four hours.

Next, an interlayer insulating film 44 was formed on the gate electrode43 and gate insulating film 40. The interlayer insulating film 44 may becomposed of a silicon nitride film, a silicon dioxide film, a siliconoxynitride film, or a stacked film of a combination of these films. Thefilm thickness may be 200 to 600 nm. In the embodiment, the filmthickness was 400 nm.

Next, contact holes are made in predetermined specific positions of theinterlayer insulating film 44. Then, a conductive layer is formed insidethe contact holes and on the surface of the interlayer insulating film44. The conductive layer is patterned into a specific shape. In thisembodiment, the source and drain electrodes 37, 38 were made of astacked film of a three-layer structure made by forming a 100-nm-thickTi film, a 300-nm-thick Ti-contained aluminum film, and a 150-nm-thickTi film consecutively by sputtering techniques. In this way, a thin-filmtransistor 35 shown in FIG. 8 was formed.

Hereinafter, an example of actually applying a thin-film transistorobtained in the embodiment to an active matrix liquid-crystal displaydevice will be explained. FIG. 10 shows an example of an active matrixdisplay device using thin-film transistors. The display device 70 has apanel structure which includes a pair of insulating substrates 71, 72and an electro-optic medium held between them. Liquid-crystal materialis widely used as the electro-optic medium 73. At the lower insulatingsubstrate 71, a pixel array section 74 and a driving circuit section areformed in an integrated manner. The driving circuit section is dividedinto a vertical driving circuit 75 and a horizontal driving circuit 76.

In the upper part of the periphery of the insulating substrate 71,terminal sections 77 for external connection are formed. The terminalsections 77 are connected to the vertical driving circuit 75 andhorizontal driving circuit 76 via wiring lines 78. In the pixel arraysection 74, rows of gate wiring lines 79 and columns of signal wiringlines 80 are formed. At the intersections of both wiring lines, pixelelectrodes 81 and thin-film transistors 82 to drive them are formed. Thegate electrodes of the thin-film transistors 82 are connected to thecorresponding gate wiring lines 79. The drain regions of the transistors82 are connected to the corresponding pixel electrodes 81. The sourceregions of the transistors 82 are connected to the corresponding signalwiring lines 80. The gate wiring lines 79 are connected to the verticaldriving circuit 75, whereas the signal wiring lines 80 are connected tothe horizontal driving circuit 76.

The thin-film transistors 82 for switching the pixel electrodes 81 andthe thin-film transistors included in the vertical driving circuit 75and horizontal driving circuit 76, which have been produced according tothe present invention, have a higher mobility than that of conventionalequivalents. Therefore, not only the driving circuits but alsohigher-performance processing circuits may be formed in an integratedmanner.

As described above, in the embodiment, since a pulse laser beammodulated so as to have a light intensity profile with a repetitivepattern where light intensity alternates between monotonous increase andmonotonous decrease is irradiated onto a non-single-crystalsemiconductor film, the high-order vibration components are decreased,which effectively suppresses the appearance of small-grain-diametercrystals caused by the vibration components. Moreover, the pulse laserbeam is caused to enter from a light-absorption heat-generation film,raising the temperature of the semiconductor film to be crystallized bythe pseudo substrate heating effect of the light-absorptionheat-generation film, which promotes the lateral crystal growth, whichmakes it possible to form crystal grains with a large grain sizeclosely.

Furthermore, even in the crystallization process, even at such a lowtemperature as room temperature and a temperature in the vicinity ofroom temperature (e.g., 5 to 50 degrees), crystallization with a largegrain size can be achieved. In addition, laser beam energy can berealized with low fluence. As for energy of an incoming laser beam forcrystallization, the crystallized region with the same lateral growthlength could be obtained by a beam with half of the energy used for thecrystallized substrate having a conventional structure or withthree-fourth of the energy used for the crystallized substrate whereonly SiO₂ was provided with a cap film.

While in the embodiment, the light-absorbable second cap oxide film 19has been an SiOx film (x is less than 2), it is desirable that x shouldbe 1.4 to 1.9. An evaluation of crystallization has shown that both ofthe crystal grain diameter and the crystal orientation had sufficientlyacceptable values when x was in the range of 1.4 to 1.9.

Furthermore, in the embodiment, for example, an XeCl excimer laser unitwhich emits a laser beam whose wavelength is 308 nm is used for theexcimer laser unit 2. However, it confirms that similar goodadvantageous effect was provided by using a KrF excimer laser unit whichemits a laser beam whose wavelength is 248 nm. It is desirable that thecrystallization process by using the laser beam whose wavelength isequal to or less than 300 nm is achieved in permitted range ofdisplacement caused by the blurring of the focal point due to heating ofthe optical system by the laser beam absorption.

Therefore, in another embodiment, a KrF excimer laser unit which emits alaser beam whose wavelength is 248 nm may be used for the excimer laserunit 2.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A crystallizing method of irradiating a laser beam onto an incidentface of a non-single-crystal semiconductor film to crystallize thesemiconductor film, the laser beam having a light intensity profileincluding a plurality of inverted triangular peak patterns in crosssection, the crystallizing method comprising: providing a cap film whichpresents absorption properties to the laser beam, on the incident faceof the non-single-crystal semiconductor film.
 2. The crystallizingmethod according to claim 1, wherein the laser beam has a wavelength of248 nm or of 300 nm or more.
 3. The crystallizing method according toclaim 1, wherein the cap film includes at least one layer of an SiOx (xis less than 2).
 4. The crystallizing method according to claim 3,wherein x in the SiOx is in the range of 1.4 to 1.9.
 5. Thecrystallizing method according to claim 1, wherein the cap film iscomposed of at least one layer of silicon dioxide film and at least onelayer of SiOx film (x is less than 2).
 6. The crystallizing methodaccording to claim 1, wherein the cap film is composed of at least twolayers of silicon oxide films differing in the relative proportions ofSi and O.
 7. The crystallizing method according to claim 6, wherein atleast one layer of the silicon oxide films is a film which presentsabsorption properties to the wavelength of the laser beam.
 8. Thecrystallizing method according to claim 1, wherein the cap film iscomposed of at least one layer of silicon dioxide film and at least onelayer of silicon oxide film differing in the relative proportions of Siand O from silicon dioxide film.
 9. The crystallizing method accordingto claim 8, wherein the silicon oxide films have a film thickness of 100to 1500 nm.
 10. The crystallizing method according to claim 1, whereinthe laser beam is a pulse laser beam modulated by a phase shifter so asto have a light intensity profile with a repetitive pattern where thelight intensity alternates between monotonous increase and monotonousdecrease.
 11. The crystallizing method according to claim 1, wherein thelaser beam is light homogenized in incident angle and light intensity.